1. Field of the Invention
The present invention relates to a piezoelectric actuator, an ink-jet head, a method of producing the piezoelectric actuator, and a method of producing the ink-jet head.
2. Description of the Related Art
An example of a piezoelectric actuator used in an ink-jet head or the like is described in Japanese Patent Application Laid-open No. 11-334087. In this piezoelectric actuator, a substrate (vibration plate) is provided such that the substrate closes openings of pressure chambers each of which communicates with a nozzle opening in a channel forming body; and a lower electrode, a piezoelectric layer, and an upper electrode are stacked in layers (laminated) on this substrate. When an electric field is applied between the upper electrode and the lower electrode, the substrate is bent with the deformation of the piezoelectric layer. Due to this, ink in the pressure chambers is pressurized and is discharged from the nozzle openings of the pressure chambers.
Such a piezoelectric actuator is manufactured, for example, as described below. First of all, a metal paste such as Pt paste is coated and then calcinated to form a lower electrode. Next, a paste containing particles of a piezoelectric material such as lead zirconate titanate (PZT) is coated on the lower electrode, and then calcinated to form a piezoelectric layer on the lower electrode (sol-gel method).
In the above-mentioned method, however, the substrate is exposed to a high-temperature environment during calcination. Accordingly, due to a difference in a coefficient of thermal expansion of materials forming the layers respectively, a stress is generated in an interface of overlapping layers, which in turn causes exfoliation of the layers. Therefore, sufficient piezoelectric characteristics cannot be achieved in some cases. Further, in some cases, an element contained in the substrate passes through the lower electrode layer and diffuses in the piezoelectric layer, thereby degrading the piezoelectric characteristics.
Namely, upon forming the electrode, it is necessary to perform calcination at a very high temperature that is not less than a melting point of the metal as the electrode material (for example, when the electrode material is Pt, not less than 900° C.) Further, upon forming the piezoelectric layer, it is necessary to perform calcination at a very high temperature that is not less than a sintering temperature of an oxide ceramics as the piezoelectric material (for example, not less than 900° C.) Furthermore, in the film formation by the sol-gel method, the upper limit for the thickness of a film formed by one step of paste coating followed by calcination is about several μm. Therefore, to form a layer having a substantial thickness, the step of coating and calcination needs to be repeated for several times. In such a case, the substrate is subjected to high-temperature environment for several times. Since the severe heat treatment is performed repeatedly during the production process, the degradation of piezoelectric characteristics due to the exfoliation of layers and/or the diffusion becomes prominent.